1. Field of the Invention
One aspect of the present invention relates to an image displaying apparatus.
2. Description of the Related Art
For an image displaying apparatus which is referred to as a projector in general, there is provided a CRT projector, a liquid crystal projector, a Digital Micromirror Device (DMD) projector, or the like, depending on the kind of an image displaying element. The DMD is a reflective image displaying element which has plural micro-mirrors arranged two-dimensionally, wherein an inclination angle of an individual micro-mirror is changed to turn on or off reflected light. An image displaying apparatus according to an embodiment of the present invention may include a reflective image displaying element such as the DMD for an image displaying element. For a publicly known technique which is to provide an image displaying apparatus with a reflective image displaying element and may be relevant to an image displaying apparatus according to an embodiment of the present invention, there is provided a technique as described in Japanese Patent Application Publication No. 2000-098272, Japanese Patent No. 3121843, Japanese Patent No. 4027712, or Japanese Patent No. 4158317, as mentioned below.
Japanese Patent Application Publication No. 2000-098272 describes an image displaying apparatus including a white light source, a color filter, a condenser lens, a plane mirror, a spherical mirror, a DMD as being one kind of a reflective image displaying element, and a projection lens being arranged in this order, the spherical mirror being arranged directly beneath an entrance portion of the projection lens, and a central axis of the condenser lens and a central axis of the projection lens intersect at a predetermined angle when viewed from a plane. An image displaying apparatus as described in Japanese Patent No. 3121843 has almost the same configuration as an image displaying apparatus as described in Japanese Patent Application Publication No. 2000-098272, wherein a folding mirror for reflecting a light ray(s) having passed through a condenser lens to be guided to a DMD is a single concave shape mirror and this folding mirror is arranged away from an optical path of reflected light which is directed from a micro-mirror of the DMD to a projection lens.
In an image displaying apparatus as described in Japanese Patent Application Publication No. 2000-098272 or Japanese Patent No. 3121843, a folding mirror for reflecting a light ray(s) having passed through a condenser lens to be guided to a DMD is arranged at an exit end side with respect to an entrance end or an end at a DMD side of a holder for holding a projection lens (referred to as a “lens barrel” below). Accordingly, a physical interference between the folding mirror and the lens barrel may be caused and the folding mirror may have to be notched extensively depending on an outer shape of a lens barrel, whereby considerable reduction in an amount of light for illuminating a DMD or an irregularity in an illuminance may readily be caused to degrade a quality of a projected image. If a notch of the spherical mirror is made small in order to solve such a problem(s), it may be necessary to decrease an aperture of a projection lens, in particular, an aperture of a projection lens between a position of the arranged spherical mirror and an end at a DMD side, whereby another problem of reduction in a speed of a projection lens or restriction on the degree of freedom of designing of a projection lens may occur.
Japanese Patent No. 4027712 describes an image displaying apparatus wherein illumination light emitted from a lamp light source enters a Total Internal Reflection (TIR) prism through a parabolic reflector, a rod integrator, and a light condensing optical system, then illumination light totally-internal reflected by the TIR prism illuminates a DMD, and light reflected by the DMD enters a projection lens through the TOR prism again.
According to an image displaying apparatus as described in Japanese Patent No. 4027712, there may be provided an advantage that it may be possible to set a reflection surface of a TIR prism near a center of a DMD and in the neighborhood of an optical axis of a projection lens. On the other hand, there may be a problem(s) that two prisms in which a precision of a surface and a precision of an angle between surfaces with respect to one another are finished by a polishing process so that high precisions may be necessary in order to constitute a TIR prism and it may be necessary to integrate such two prisms at a high precision whereby its cost may be higher than the case where an optical system for illuminating a DMD is configured by using a mirror. Furthermore, an optical path is folded by means of total reflection inside a TIR prism but such a TIR prism has loss of reflection at many surfaces such as (1) an entrance surface of a prism, (2) an exit surface of a DMD for irradiation light, (3) a reflected light entrance surface of a DMD, (4) a boundary surface of a prism (at an exit side), (5) a boundary surface of a prism (at an entrance side), and (6) an exit surface of a prism, which may be a factor of reduction in efficiency of use of illumination light. Moreover, a mass of a TIR prism may be greater than that of a mirror, and hence, may be a factor inhibiting attainment of a lightweight image displaying apparatus.
Japanese Patent No. 4158317 describes a projection-type displaying apparatus including an illumination device in which a luminous body, light condensing means, light mixing means, a first lens group, a second lens group, and a light valve as being one kind of a reflective image displaying element are arranged in this order, and further including a projection lens (corresponding to a projection optical system) for transmitting light emitted from the light valve to a screen.
In a displaying apparatus as described in Japanese Patent No. 4158317, a part of a second lens group which is one of optical components constituting an illumination optical system is also shaded by a lens barrel for a projection lens and a part of a second lens group is notched more extensively than an outer shape of the lens barrel, like an invention as described in Japanese Patent Application Publication No. 2000-098272 or Japanese Patent No. 3121843. Accordingly, an irregularity in an illuminance of illuminated light on a reflective image displaying element may readily be caused to degrade a quality of a projection image.